Wireless ranging may be used to determine a range between two wireless units. This may be helpful information for firefighters working on different floors or rooms in a building, for example. The Global Positioning System (GPS) is a common wireless position determining system that can be used to determine a position on the earth. The GPS position information could be reported to another firefighter; however, GPS has line-of-sight requirements that prevent the system from working reliably underground or in other enclosed spaces.
A typical form of wireless ranging uses time-of-arrival (or time-of-flight) processing to determine a range between two wireless units. For example, if the units are time synchronized, a first unit can include a time-of-arrival transmitter to send a signal to a time-of-arrival receiver at a second unit. A processor at the second unit can determine a range between the units based on the time required for the signal to travel between the units. Unfortunately, a time-or-arrival signal may be reflected off of near-by surfaces so that multi-path interference at the receiver reduces the accuracy of range determination.
U.S. Pat. No. 6,054,950 to Fontana discloses a wireless ranging system that includes an array of ultra-wideband (UWB) transceivers that transmit signals to a node having an UWB receiver and a processor. The processor calculates time-of-flight measurements of the signals received from the UWB array of transceivers to determine the node's geolocation. Similarly, U.S. Pat. No. 5,589,838 to McEwan discloses a wireless transmitter that transmits a signal to an array of receivers connected to a processor. The processor measures the time-of-flight of the signal's arrival at each individual receiver to determine the transmitter's location.
U.S. Pat. No. 6,133,876 to Fullerton et al. discloses an UWB impulse radio ranging system including a first transceiver having a first clock that generates a first reference signal, and a second transceiver having a second clock, which generates a second reference signal. The first reference signal is used to synchronize the second transceiver and the second reference signal is used to synchronize the first transceiver. A delayed first reference signal is then transmitted and the difference between the first reference signal and the delayed first reference signal is used to determine the distance between the first and second transceivers.
Another type of wireless ranging system is disclosed U.S. Patent Application Publication No. 2004/0032363 to Schantz et al. This patent discloses near-field electromagnetic ranging using a receiver and processor to determine the phase difference between the electric field and the magnetic field in an electromagnetic wave transmitted from received from another wireless unit. Unfortunately, the operating frequency needs to bear a predetermined relationship to the actual range.
Unfortunately, the prior art wireless ranging systems each have some shortcomings. For example, GPS suffers from line-of-sight restrictions. Time-of-arrival ranging suffers from multi-path interference in certain environments. Near-field electromagnetic ranging requires that the operating frequency be properly related to the actual range.